Fluorescent lamps are increasingly used because of their high efficiency in converting electrical energy into light. Ballasts are used to operate gas discharge lamps by supplying controlled power to heat a lamp's cathodes or filaments and to supply sufficient starting or striking voltage to ionize the gas and establish an arc between the lamp's filaments.
One important class of ballasts is the "rapid start" ballast. In a rapid start ballast, filament or cathode voltage is first applied before striking an arc through the lamp, which takes about 750 milliseconds. This mode of operation provides optimum performance and best lamp life based on number of cold starts. One such ballast is disclosed in U.S. Pat. No. 5,144,195 to Konopka et. al. and assigned to the assignee of the present invention. In that patent a ballast is disclosed wherein filament heating is controlled by delaying the boost startup with respect to the lamp drive inverter. The inverter starts immediately when power is applied and the boost starts about 700 milliseconds later. Until the boost starts, the inverter has insufficient voltage output to strike the lamps. During this time the output voltage is sufficient to heat the filaments. When the boost comes on, the voltage to the inverter rises, lamp voltage rises and the lamps are struck.
A more modern ballast is disclosed in U.S. Pat. No. 5,399,944 to Konopka et. al. and assigned to the assignee of the present invention. U.S. Pat. No. 5,399,944 discloses a "one transistor" ballast. That ballast comprises an energy storage circuit, a power transistor switch that is operated in response to an oscillator and a resonant circuit that couples the energy storage circuit to the fluorescent lamp. Only one power transistor is used for the entire operation of the circuit, compared to two or three transistors that are used in ordinary power factor corrected ballasts. Not only is the cost of manufacture reduced but also the ballast's energy storage capacitors operate at a voltage just slightly less than the peak of the line voltage. This is advantageous compared to many other ballast circuits that require energy storage capacitors to operate at voltages well above the peak of line voltage.
Although the "single transistor" ballast represents an important development in the art, further improvements can be made. In particular, it would be highly desirable to improve lamp life by providing for pre-heating of the lamp filaments before striking the arc through the lamp .